The irrigation technique is well known by those skilled in the art. It consists in artificially bringing water to crop soil. It is used inter alia for crops with high water consumption (e.g., corn, cotton, etc.) or in zones with little rainfall, in order to meet the needs of the plant.
Irrigation methods for example include gravity-fed irrigation (by flooding or using lines), spray irrigation (with rollers, moving bars or pivots) and localized irrigation, but are not limited to these examples alone.
Localized irrigation encompasses a set of irrigation methods making it possible to supply water precisely, generally at lower water flow rates and from stationary equipment. Drip irrigation, porous hose irrigation and irrigation using stationary micro-sprayers are examples of localized irrigation.
Drip irrigation is an irrigation method that implements drip pipes making it possible to supply water at a very low flow rate. The name “drip” does not, however, limit this method to supplying water only in the form of drops of water. It is in fact possible to consider a very low water stream (running water).
Drip pipes are devices provided with emission orifices. They may be thin simple pipes, also called capillaries, wherein the diameter and length thereof are chosen such as to generate a sufficient pressure loss in light of the pressure of the water in the supply grid to obtain a lower water flow rate, generally below 10 liters per hour. Drip pipes may also assume the form of more sophisticated flow rate regulating devices, generally provided with narrow baffles, sometimes making it possible to obtain a fixed flow rate within a given pressure range.
Under agricultural production conditions, drip irrigation systems generally assume the form of pipes provided with orifices having equipment making a possible to control the flow rate of the water through this orifice for given pressures (example: UNIRAM® or DRIPCORN® equipment provided by the company NETAFIM).
Typically, the localized irrigation methods lead to an irrigation bulb positioned near each water emission point. This bulb appears within the first moments following the startup of the irrigation (the irrigation bulb corresponds to the moistened zone, p. 46 in the Irrigation Handbook, Cemagref Edition).
The aim of these irrigation methods is to ensure that the moistening bulbs result in being joined together, making it possible, from one to the next, to achieve homogeneous moistening of the arable layer. Indeed, a deep infiltration of the water is not beneficial to the crops. It follows that the zones not receiving irrigation water between the emission points would represent a proportional amount of less productive surface area. Increasing the diameter of the irrigation bulb also makes it possible to separate the water emission points. This results in a lower cost per surface unit of a localized irrigation installation.
To allow for better distribution of the water, document U.S. Pat. No. 6,343,749 proposes moving irrigation systems in the form of drip lines moving over the surface of a parcel, for example using a pivot. However, this method requires heavy equipment demanding considerable investments and energy. Furthermore, the emitting members travel paths parallel to one another. The absence of overlap of the traveled zones causes non-homogeneous moistening of the arable layer.
Drip irrigation systems can also be buried. In the case of a corn crop, for example, the drip pipes are generally buried between 20 and 60 cm deep. This type of installation makes it possible to leave the installation in place, without having to move or remove the pipes on the surface when working the field. In the case of a buried drip irrigation system, the rising of the water to the surface is an important point, making it possible to do without a surface irrigation system.
Acrylamide-based polymers can be added to the irrigation water for various benefits, such as decreasing water and wind erosion, controlling rain damage or controlling water infiltration in the soil. The Natural Resources Conservation Service at the USDA concisely describes this use of acrylamide-based polymers added to the irrigation water within the Conservation Practice Standard Code 450.
The polymers used to stabilize the soil are polymers with a high molecular weight (greater than 1 million Daltons). As an example, the document EP 1,105,443 describes soil treatment compositions that may in particular be beneficial in the stabilization of soils and that may be used in drip irrigation methods. The compositions are aqueous solutions comprising, inter alia, a water-soluble anionic polymer having an intrinsic viscosity of 9 to 12 dL/g (at 20° C., in a solution of NaCl 1 M at pH 7.0). It is mentioned that the polymer has a high enough molecular weight to provide a soil stabilization effect. It is also specified that it does not have a low molecular weight, which would otherwise cause it to act as a dispersant.
Such polymers can be used in localized irrigation systems. For example, Shane Phillips (Master's Thesis, 2007, Effect of Polyacrylamides on the Physical Properties of Some Light-textured Soils, University of Adelaide) describes drip irrigation systems.
The addition of these polymers to the irrigation water is performed using emulsions, dispersions or a powdered product previously dissolved in a parent solution.
The aqueous dispersion polymers (dispersion in saline solution) have a high salt content and therefore have a risk of clogging the outlet orifices for the irrigation water (drip pipes, pores, micro-sprayers).
The emulsions can be used to prepare a parent solution or be injected directly in the water grid that supplies the drip pipes. Document WO1998/057531 thus describes the injection of the polymer in emulsion form in a pressurized irrigation system.
The polymers can be added to the irrigation water in powder form, for example in the case of a gravity-fed irrigation system. However, the powder form is impractical to handle, since it causes dust formation, which is dangerous for the user. Furthermore, this form requires a mixing apparatus and a substantial dissolution time. When the dissolution is not done correctly, it generates lumps that then plug the drip pipes of a drip system. Lastly, the maximum soluble polymer concentration within the water is limited for polymers with a high molecular weight.
Using an inverse emulsion of acrylamide-based polymers with a high molecular weight requires being able to reverse this emulsion correctly and to dilute it in water quickly. In the case, for example, of a large spray irrigation system such as a pivot, the water volumes and flow rate used facilitate the reversal of these emulsions. In the case of localized irrigation systems such as drip irrigation systems, the water pressure and flow rate are low, making the reversal of the emulsion injected into the water supply grid random. If the reversal is performed incorrectly, a lump of polymer forms and can plug the pipes, the micro-sprayers or the drip pipes, for example. Furthermore, the reverse emulsions of acrylamide-based polymers generally contain solvents, such as mineral oils, that are incompatible with certain materials used in localized irrigation systems: PVC, low-density polyethylene, elastomers, etc.
Another solution may be to use polymers in solution. However, polyacrylamides with a high molecular weight in solution are then at low concentrations (with a maximum polymer concentration of 5 to 10% by weight). This would therefore create logistical problems due to the very large quantities of solution to be transported and handled.
By nature, polyacrylamides with a high molecular weight are flocculant. This is also why they are used in agriculture. Their flocculant power stabilizes soil and limits the particles in suspension, thus decreasing erosion. In a localized irrigation system like a drip irrigation system, the flow rates are low and the water emission orifices are narrow. The presence of particles in suspension in the water, even in a low concentration, causes, in contact with the polyacrylamide, the formation of flocks that clog the irrigation water outlet orifices (drip pipes, micro-sprayers, pores), or even the water supply grid itself.
To resolve this problem, an attempt could be made to add the polymers based upon acrylamides with a high molecular weight upstream from a filtration system, for example a sand filter. This would result in removing a non-negligible part of the previously added polymers from the irrigation water, by retaining the flocks or adsorbing polymers, rendering the entire method pointless.
Document U.S. Pat. No. 3,633,310 describes a method making it possible to improve the irrigation of highly permeable soil. This method consists in adding irrigation water, a small quantity of polyacrylamide with a molecular weight between 300,000 and 15 M, advantageously between 500,000 and 5 M. The polyacrylamide makes it possible to make any portion of the irrigated soil less permeable for subsequent irrigations (without adding polyacrylamide). No irrigation method is mentioned.
Document U.S. Pat. No. 5,120,344 describes a method consisting of applying a layer of gel at a small depth on the surface of the soil to be irrigated to improve the retention of water and nutrients. This layer of gel is obtained from a clay and a polymer having a molecular weight between 50,000 and 20 M Da, preferably between 500,000 and 10 M Da. The gel is formed before being introduced into the soil or “in situ” in the soil. In all cases, the layer of gel is applied before any irrigation.
Document US 2009/0239973 A1 describes an irrigation system implementing polyacrylamide with a molecular weight between 10,000 and 100,000 Da. Several embodiments are proposed. The first method consists in spraying irrigation water on a polyacrylamide block. The second method consists in positioning the polyacrylamide block within channels wherein the irrigation water circulates. This is not a localized irrigation method.